483
THE EFFECT OF SALINITY LEVELS ON THE STRUCTURE
OF ZOOPLANKTON COMMUNITIES
Ewa Paturej and Agnieszka Gutkowska*
Department of Applied Ecology, University of Warmia and Mazury in Olsztyn, Oczapowskiego 5, 10-957 Olsztyn, Poland
*Corresponding author: [email protected]
Abstract:The objective of this study was to determine the qualitative and quantitative structure of zooplank-ton communities in the Vistula Lagoon and to establish whether zooplankzooplank-ton abundance and biodiversity are affected by salinity levels. Samples for biological analyses were collected in the summer (June-September) of 2007-2011 at eleven sampling sites. Statistical analysis revealed a significant correlation between salinity levels and the number of species (r= -0.2020), abundance (r= 0.1967) and biomass (r= 0.3139) of zooplankton. No significant correlations were found between salinity and the biodiversity of zooplankton. The results of the study suggest that salinity affects the abundance and structure, but not the diversity of zooplankton communities in the Vistula Lagoon.
Key words: zooplankton abundance; biological diversity; brackish waters; Vistula Lagoon
Received September 10, 2014; Revised November 6, 2014; Accepted November 13, 2014
INTRODUCTION
Brackish water habitats are characterized by highly unstable environmental conditions, pri-marily salt concentrations (Cognetti and Malta-gliati, 2000). They differ from marine and inland waters in terms of salinity levels, hydrological and ecological conditions, which is why they continue to arouse interest among researchers. The mixing of saline water with freshwater inflows leads to salinity and temperature changes that affect the biotic and abiotic components in the water body. The organisms living in brackish waters have to be able to tolerate rapid drops and fluctuations in
number of taxa occur at critical salinity levels of 5-8 PSU, which is referred to as the “paradox of brackish waters”. The highest decline is observed in the number of Rotifera and Oligochaeta species of freshwater origin. The overcoming of the criti-cal salinity barrier was an important evolutionary event and a major step towards the colonization of freshwater habitats. Changes in salinity lead to increased mortality of both freshwater species adapted to brackish water environments and ma-rine species adapted to freshwater environments. An adaptive response of zooplankton to adverse environmental conditions may involve distur-bances in reproduction and development (Hart et al., 2003; Silva et al., 2009; Santangelo et al., 2014).
The aim of this study was to determine the qualitative and quantitative structure of zoo-plankton communities in the Vistula Lagoon and to verify the hypothesis that the number of species, abundance, biomass and biodiversity of zooplankton are affected by salinity levels.
MATERIALS AND METHODS
The study was conducted on the Vistula Lagoon situated in the southern coast of the Baltic Sea. The Vistula Lagoon is a large, but shallow wa-ter body. The Polish part of the lagoon has a to-tal area of 328 km2, an average depth of 2.6 m and a maximum depth of 4.4 m (Chubarenko and Margoński, 2008). The Vistula Lagoon is a brackish water body due to the mixing of saline water from the Baltic Sea with freshwater inflows from rivers that feed into the lagoon. Regarding dissolved salt concentrations, the Polish part of the Vistula Lagoon can be divided into two re-gions: a region close to the river mouth (Western Basin), dominated by fresh water (0.5-2 PSU, up to 4 PSU during storms) and the central region
(Central Basin), dominated by saline water (2-4 PSU, up to 6 PSU during brackish water inflows) (Kruk et al., 2012; Witek et al., 2010).
Samples for biological analyses were collect-ed once a month, in the summer season (June-September) of 2007-2011 at eleven sampling sites (Fig. 1), with a Ruttner sampler and a 10 dm-3 bucket at shallow sites in the coastal zone. The biological material (30 dm-3) was concentrated using an Apstein plankton net (30 μm), and was fixed in Lugol’s solution followed by a 4% for-malin solution. A total of 108 samples were col-lected. Salt concentrations were measured with a WTW Multi 350i multi-meter.
Qualitative and quantitative determinations of zooplankton were made based on a micro-scopic analysis of the experimental material. For the purpose of a qualitative assessment, zoo-plankton were identified to the species level with-in three taxonomic groups: Rotifera, Cladocera and Copepoda, and the Copepoda were further subdivided into developmental stages of nauplius and copepodid. A quantitative analysis, including zooplankton abundance and biomass, was per-formed as recommended by Bottrell et al. (1976); Ejsmont-Karabin (1998); Hillbricht-Ilkowska and Patalas (1967) and Starmach (1955).
Table 1. List of recorded zooplankton species in the Vistula Lagoon over the experimental period.
2007 2008 2009 2010 2011
Anuraeopsis fissa (Gosse, 1851) + + + + +
Ascomorpha saltans Batrsch, 1870 + + + +
Asplanchna priodonta Gosse, 1850 + + + + +
Brachionus angularis Gosse , 1851 + + + + +
Brachionus calyciflorus Pallas, 1766 + + + + +
Brachionus diversicornis Daday, 1883 +
Brachionus quadridentatus Hermann, 1783 + +
Brachionus urceolaris Müller, 1773 + + +
Cephalodella catellina Müller, 1786 + +
Cephalodella gibba Ehrenberg, 1832 + +
Colurella colurus Ehrenberg, 1830 + + + + +
Colurella uncinata Müller, 1773 +
Euchlanis dilatata Ehrenberg, 1832 + + + +
Filinia brachiata Rousselet, 1901 +
Filinia longiseta Ehrenberg, 1834 + + + + +
Hexarthra mira (Hudson 1871) +
Keratella cochlearis (Gosse,1851) + + + + +
Keratella quadrata (Müller, 1786) + + + + +
Lecane closterocerca (Schmarda, 1859) + +
Lecane flexilis (Gosse, 1886) + +
Lecane luna (Müller, 1776) + +
Lepadella ovalis (Müller, 1786) + +
Mytilina mucronata (Müller, 1773) +
Polyarthra longiremis Carlin, 1943 + +
Polyarthra platyptera Ehrenberg, 1838 + + +
Polyarthra vulgaris Carlin, 1943 + + +
Pompholyx sulcata Hudson, 1885 + + + + +
Proales sp. + +
Rotaria neptunia (Ehrenberg, 1832) + +
Rotaria rotatoria (Pallas, 1766) + +
Synchaeta sp. + + + + +
Testudinella elliptica (Ehrenberg, 1834) + +
Testudinella patina (Hermann, 1783) +
Trichocerca cylindrica (Imhof, 1891) +
Trichocerca pusilla (Lauterborn, 1898) + + + + +
Trichocercasimilis (Wierzejski, 1893) + + + +
Trichotria pocillum (Müller, 1776) +
Trichotria tetractis (Ehrenberg, 1830) +
Alona rectangula G.O. Sars, 1862 + +
2007 2008 2009 2010 2011
Bosmina longirostris (Müller, 1785) + + +
Cercopagis pengoi (Ostroumov, 1891) +
Ceriodaphniaquadrangula (Müller, 1785) + + +
Chydorus sphaericus (Müller, 1776) + + +
Diaphanosomabrachyurum (Liévin, 1848) + + + + +
Eubosmina coregoni (Baird, 1857) + +
Leptodora kindtii (Focke, 1844) + +
Sida crystallina (Müller, 1776) +
Acanthocyclops vernalis (Fischer, 1853) +
Acartia bifilosa (Giesbrecht, 1881) + +
Acartia longiremis (Dana, 1846) + + + + +
Acartia tonsa (Dana, 1846) + + + + +
Cyclops sp. + + + +
Eurytemora affinis (Poppe, 1880) + + + + +
Harpacticoida + +
Megacyclops viridis (Jurine, 1820) +
Mesocyclops leuckarti (Claus, 1857) + +
Metacyclops gracilis (Lilljeborg, 1853) +
Paracyclops affinis (G.O. Sars, 1863) +
Temora longicornis (Müller, 1785) +
Thermocyclops oithonoides (G. O. Sars, 1863) +
kopepodit Calanoida + +
kopepodit Cyclopoida +
nauplii Calanoida + + + + +
nauplii Cyclopoida + + + + +
nauplii Harpacticoida + +
The zooplankton structure was characterized using the following biocenotic indicators: species constancy (Tischler, 1949 cited by Trojan, 1980), species domination (Kasprzak and Niedbała, 1981), the Shannon diversity index (based on abundance and biomass), (Shannon, 1948) and Pielou’s evenness index (Pielou, 1974).
The results obtained were processed statisti-cally with the use of STATISTICA PL 10.0 soft-ware. The effect of salinity levels on the qualita-tive and quantitaqualita-tive structure of zooplankton was estimated using Pearson’s correlation coefficient.
RESULTS
During the study, salinity in the Vistula Lagoon varied over a wide range, subject to sampling site and date. In 2007, salinity ranged from 2.4 to 9.3 PSU, in 2008 from 2.6 to 4.4 PSU, and in 2009 from 1.8 to 4.4 PSU. In the next two years, salinity did not exceed 4.0 PSU. Average salinity in 2010 and 2011 reached 2.1 PSU and 1.5 PSU, respectively.
The zooplankton of the Vistula Lagoon com-prised three taxonomic groups: Rotifera, Cladoc-era and Copepoda (Table 1). The avCladoc-erage number
of recorded species was 9 in 2007 and 2008, 10 in 2009 and 14 in 2010 and 2011. They were mostly freshwater organisms with a wide tol-erance to salinity and a few brackish and ma-rine taxa. In all years of the study, the qualita-tive structure of zooplankton was dominated by the rotifers Filinia longiseta and Keratella
co-chlearis, accompanied by Brachionus angularis
in 2007, 2010 and 2011. Furthermore, in 2010
Polyarthra longiremis, Pompholyx sulcata and
Bosmina longirostris formed a high proportion
of the zooplankton community (Table 2). Over the experimental period, 17 constant species were recorded in the Vistula Lagoon. Keratella cochlearis and Filinia longiseta were absolutely constant species, whereas constant species were much more numerous (Table 3).
A quantitative analysis of the collected sam-ples revealed that the average densities of zoo-plankton in the Vistula Lagoon ranged from 1235 to 5704 ind. dm-3 (Fig. 2). The Rotifera accounted for 78.4-94.2% of the total zooplankton abun-dance (Fig. 3).
Keratella cochlearis (9662-63007 ind. dm-3),
Filinia longiseta (18-17185 ind. dm-3) and mem-bers of the genus Brachionus (675-4876 ind. dm-3) reached the highest total abundance among roti-fers in the studied years. Crustaceans had a 5.8-21.6% share of the total zooplankton abundance.
Among cladocerans, Diaphanosoma brachyurum
(104-1654 ind. dm-3)and Bosmina longirostris (152-2084 ind. dm-3) were characterized by high abundance values. Among copepods, high abun-dance values were reported for Eurytemora affinis
(135-1498 ind. dm-3) and the juvenile stages of crustaceans - nauplii (1960-4693 ind. dm-3). The abundance of copepods was also indicated by the presence of Acartia longiremis (558-1468 ind. dm-3) and Acartia tonsa (186-1102 ind. dm-3).
Average zooplankton biomass ranged from 0.94 to 18.40 mg/dm3 (Fig. 4), mostly due to the presence of crustaceans (Fig. 5) whose share of the total biomass reached 63.3-97.3%.
Among Cladocera, Diaphanosoma brachy-urum was characterized by the highest total biomass (3.56-85.59 mg dm-3) in the studied years. Leptodora kindtii, whose occurrence was seasonal, also contributed greatly to an increase in zooplankton biomass. The Copepoda,
includ-ing Eurytemora affinis (2.75-68.86 mg dm-3)
and Acartia longiremis (20.15-36.11 mg dm-3),
also formed a high proportion of the total zoo-plankton biomass. Rotifera had a low percent-age share of zooplankton biomass, except for
Keratella cochlearis (0.89-18.83 mg dm-3) and
representatives of the genus Brachionus (0.36-4.00 mg dm-3), which dominated also in terms of abundance.
Table 2. Eudominant and dominant zooplankton species* in the Vistula Lagoon over the experimental period.
2007 2008 2009 2010 2011
Brachionus angularis 9.5 10.3 13.3 Filinia longiseta 5.3 10.2 20.7 10.2 18.8 Keratella cochlearis 53.8 67.9 65.0 30.6 37.7
Polyarthra longiremis 17.0
Pompholyx sulcata 5.1
Bosmina longirostris 6.6
The biological diversity of zooplankton com-munities in the Vistula Lagoon was estimated based on Pielou’s evenness index and the Shan-non diversity index calculated using abundance and biomass values. Pielou’s evenness index re-mained in the 0.24-0.72 range, indicating that the community was not even. The Shannon diversity index based on zooplankton abundance reached 0.6-2.1. The relatively low values of this index resulted from a clear dominance of one species. For instance, in August 2007, the Shannon di-versity index was determined at 0.6, and Kera-tella cochlearis was the dominant species, with an
83.5% share of the total zooplankton abundance. A similar situation was encountered in June and July 2009, when the value of the index was also 0.6, and Filinia longiseta (83.2%) and Keratella cochlearis (83.5%), respectively, dominated in the zooplankton community.
The Shannon diversity index calculated based on zooplankton biomass remained in the 0.9-2.8 range. Similar to abundance, the minimal value of the index was noted when one taxon clearly dominated over the others (H’ = 0.9 –
Euryte-mora affinis – 68.8%, H’ = 1.0 – Diaphanosoma
brachyurum – 64.6%)
There were significant correlations between sa-linity levels, the number of species, abundance and biomass of zooplankton. No significant correlations were found between salinity and the biodiversity of zooplankton (measured as the Shannon diver-sity index) based on abundance (Hl) and biomass (Hb), and Pielou’s evenness index (e), (Table 4).
DISCUSSION
Lagoons are shallow bodies of water with both freshwater and saltwater inflows. During seawater intrusions, marine processes dominate over flu-vial processes in entire estuaries or their parts. In the absence of saltwater inputs, the physicochem-ical properties of estuarine water are similar to those observed in extensive inland water bodies (Paturej, 2005; Paturej and Bogacka, 2001). Salin-ity may vary over a wide range (0.05-35 PSU) in lagoons, depending on the inflow of freshwater and seawater, wind direction and their individual characteristics (Paturej, 2006). Differences in salt concentrations affect the ecological processes in lagoons, including zooplankton abundance and diversity (Badsi et al., 2010; Dube et al., 2010; Gao
Fig. 2. Average densities [ind. dm-3] of zooplankton in the Vistula
Lagoon during the study period.
et al., 2008; Paturej, 2009). Changes in salinity may directly and indirectly influence zooplankton abundance, leading to the extinction of some spe-cies and the appearance of others. Many organ-isms migrate to avoid high or low salinity. Salinity fluctuations may indirectly cause or contribute to food shortage, thus affecting zooplankton abun-dance (Perumal et al., 2009).
In the Vistula Lagoon, differences in salinity are caused by seawater intrusion or reduced fresh-water inflow when the fresh-water level is low (Kruk et
al., 2012). Therefore, primarily organisms with a wide tolerance to salinity fluctuations occur in an-imal plankton of the Vistula Lagoon. In the pres-ent study, we found mainly freshwater, euryhaline Rotifera, such as the genera Brachionus, Keratella cochlearis or Filinia longiseta. Among copepods mainly Calanoida – typical brackish water
Eu-rytemora affinis and marine species of the genus
Acartia (A. tonsa and A. longiremis), occur. Cla-docerans appeared occasionally, on the freshwater sites, which is characteristic for this group, as they are typically freshwater organisms, and most
spe-Table 3. Absolutely constant and constant species* in the Vistula Lagoon over the experimental period.
2007 2008 2009 2010 2011
Acartia longiremis 85 85 80
Acartia tonsa 60
Eurytemora affinis 75 75 58
Asplanchna priodonta 54
Brachionus angularis 100 79 83
Brachionus calyciflorus 55 58
Brachionus urceolaris 54
Colurella colurus 63
Euchlanis dilatata 54 67
Filinia longiseta 85 100 63 96
Keratella cochlearis 85 100 100 100 100
Lecane closterocerca 54
Polyarthra longiremis 67
Pompholyx sulcata 83 71
Polyarthra vulgaris 60 Polyarthra platyptera 70
Trichocerca pusilla 70 75 60 67 100
*Frequency criterion according to Tishler (1949) as cited in Trojan (1980): 100-76% (absolutely constant species), 75-51% (constant species)
Table 4. Correlations between salinity and the values of zooplankton structure and biodiversity indicators.
Number of species Abundance [ind./dm3] Biomass [mg/dm3] Hl Hb E
cies do not tolerate high salt concentrations (Boix et al., 2007; 2008; Brucet et al., 2009). Thus, even a slight increase in salinity reduces the abundance and richness of cladocerans. Their disappearance in saline waters promotes the development of ro-tifers, in particular euryhaline species of the gen-era Brachionus and Colurella, as representatives of both groups are mostly filtrators and compete for food (Anton-Pardo and Armengol, 2012). Thus, brackish waters are characterized by the domi-nation of small organisms, such as rotifers and small-sized crustaceans (Brucet et al., 2009; 2010; Jeppesen et al., 1994).
Salinity affects not only the structure of zoo-plankton, but also its abundance. In the present
study, a significant, positive correlation was ob-served between zooplankton density and bio-mass, and salinity in the Vistula Lagoon, which was probably related to the increased propor-tion of copepods in the structure of zooplank-ton, especially species such as Eurytemora af-finis, Acartia tonsa and Acartia longiremis. The appearance of copepods, with an increase in salinity that resulted in an increase in abun-dance of planktonic fauna, has been observed also by Badsi et al. (2010); Gao et al. (2008)
and zakaria et al. (2007). The increase in sa-linity may also cause a decrease in the species richness. In the Vistula Lagoon, the number of zooplankton species was negatively corre-lated with salinity, which can be explained by the occurrence of the brackish water paradox phenomenon, attributable to the salinity of 5-8 PSU. Similar observations were made by zorina-Sakharov et al. (2014) in the fore-delta shallow water habitats of Kyliya branch of the Danube River. The structure of zooplankton was domi-nated by freshwater species, but species rich-ness decreased linearly with the increase of sa-linity. Nonetheless, Telesh et al. (2011) claim that planktonic organisms often do not show minimum species diversity within the marine-freshwater salinity gradient. zooplankton biodi-versity also depends on salinity, as evidenced by Echainz and Vignatti (2011) and Schallenberg et al. (2003), an an increase in salinity causes loss of biodiversity. This study, however, does not support this thesis, probably because salinity fluctuations were too small to cause changes in the diversity of zooplankton, or the study period was too short to capture changes in biodiversity.
It may be concluded that salinity is a key fac-tor affecting the structure of zooplankton munities. In the Vistula Lagoon, the species
com-Fig. 4. Average zooplankton biomass [mg dm-3] in the Vistula
Lagoon during the study period.
position and total abundance of zooplankton are determined by salinity. A small increase in salin-ity, observed in the examined water body, leads to a decrease in species richness as well as an increase in the abundance and biomass of zoo-plankton. According to literature data, a rise in salt concentrations causes changes in biodiversity measured with the use of biocenotic indicators. However, such changes were not noted in our study. Therefore, further research is needed to verify whether zooplankton biodiversity in the Vistula Lagoon is influenced by salinity.
Acknowledgments: The study was carried out as part of the research project System of environmental and spatial information as the background for sustainable management of the Vistula Lagoon ecosystem (VISLA) financed by the Polish-Norwegian Research Fund.
Authors’ contributions: Study Design-Agnieszka Gut-kowska and Ewa Paturej; data Collection-Ewa Paturej; statistical Analysis-Agnieszka Gutkowska; data interpre-tation-Agnieszka Gutkowska and Ewa Paturej; manuscript preparation-Agnieszka Gutkowska; literature search-Ag-nieszka Gutkowska; funds collection-Ewa Paturej.
Conflict of interest disclosure: The above-mentioned manuscript has not been published before and is not under consideration for publication anywhere else. The publica-tion of this article was approved by both authors, as well as by the responsible authorities.
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